(263a) Towards A Univeresal Phase Diagram For Functional Rod-Coil Block Copolymers

Emerging applications for polymers in organic photovoltaics and biotechnology require the patterning of materials on the 10-100 nm length scale, and block copolymers provide an elegant route to self-assemble such nanostructured phases. However, the rodlike nature of many functional polymers, such as helical proteins or conjugated semiconducting polymers, complicates self-assembly through changes in chain topology and the interplay between liquid crystalline interactions and microphase separation. To understand self-assembly in these systems, we have prepared a model rod-coil diblock copolymer with accessible phase transitions and used it to chart the phase diagram for these materials.

Using a series of 22 different weakly and moderately segregated rod-coil diblocks, we have developed a phase diagram for this class of materials. By side chain functionalizing the rod block to appear chemically similar to the coil, the rod-rod and rod-coil interactions are reduced, allowing both order-disorder and liquid crystalline phase transitions to be accessed. Liquid crystalline aligning interactions between the blocks promote the formation of lamellar phases across most of the ordered region of the phase diagram, with heating above the order-disorder transition resulting in both nematic and isotropic phases. At high coil fraction and high asymmetry in size between the rod and coil, hexagonal arrangements of rectangular rod nanodomains are also observed.

Independent measurements of the temperature dependence of the rod-rod and rod-coil interaction parameters allow us to convert this system-specific phase diagram to a universal phase diagram for all polymers with a rod-coil molecular structure. Measurements of the nematic-isotropic transition in rod homopolymers as a function of molecular weight allow estimation of the Maier-Saupe rod-rod interaction parameter for nematic liquid crystals. These measurements show that both enthalpic and entropic contributions to this parameter are important. The Flory-Huggins rod-coil interaction parameter is measured through interfacial segregation of a rod-coil block copolymer to the interface between rod and coil homopolymers. Self-consistent field theory is used to model the interfacial excess of block copolymer as a function of the rod-coil interaction strength, allowing the rod-coil interaction parameter to be estimated by comparison with experimental results.